Discover the progressive evolution of sex chromosomes

Discover the progressive evolution of sex chromosomes
© iStock/BlackJack3D

University of Konstanz published new findings of the evolution of sex chromosomes that ran for 30 years on the genomic mechanisms of sex determination in fish.

The laboratory of Professor Axel Meyer, University of Konstanz, Germany, published the new findings of an experimental evolutionary project looking specifically at the evolution of sex chromosomes in swordtail fish.

The diversity of sex

The diversity of sex determination systems is unusually varied among fishes compared to mammals that all have a stereotypic XX, XY sex-chromosomal mechanism to determine sex. Why and how the genomic mechanisms and the evolution of sex chromosomes causes such diversity in sex determination in fish are so variable remained unknown.

Evolutionary biologist Dr Paolo Franchini, Junior Research Group Leader at the University of Konstanz, with his collaborators has now been able to describe such a mechanism and its exact function in this study.

The experiment was initiated by including hybridisation with backcrossing two Xiphophorus fish species with different sex chromosome systems. Hybrid origins of species, the result of the crossing of two genetically different species, occur frequently in fish populations.

“What we know from the study of these systems in natural habitats is that hybridisation is a crucial mechanism for the evolution of new species”, explains Franchini. In the experiment the researchers could study the effects of hybridisation on the genomic mechanism of sex-determination.

Determining the sex of these fish is relatively easy: as its name suggests, the male swordtail fish has a sword-like elongated caudal fin; the female, on the other hand, is rounder and fuller and does not have a swordtail. In this long-term laboratory study, the effects of hybridisation that is thought to have resulted in at least two swordtail species in nature on the genome and the mechanisms that determine sex could be studied in detail.

The evolution of sex chromosomes

In an experimental design that makes it possible to track crossings of more than 100 generations over more than 30 years, a female Xiphophorus maculatus was crossed repeatedly with a male Xiphophorus hellerii. These two species have different sex chromosome systems. In further steps, the new hybrid fish originating from the cross was backcrossed again with the male species, and this design was repeated for more than 100 generations.

Franchini explains further: “We found out that introgression – the movement of genetic material from one species to another – and the selection of pigmentation phenotypes results in the retention of an unexpectedly large maternally derived genomic region.”

During the hybridisation process, the sex-determining region on the X chromosome of one parent was transferred to an autosome of the hybrid fish. This transfer led to the evolution of sex chromosomes – and most importantly an entirely new sex chromosome.

Franchini concludes: “Our results show above all the complexity of factors that contribute to patterns observed in hybrid genomes including such fundamental issues such as sex determination”.

The work proves that hybridisation can instigate the rapid evolution of a new sex chromosome and thus makes an essential contribution to an experiment launched more than 30 years ago.

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